The present invention relates to a photocurable adhesive composition and its use in the optical field.
It is a common practice in the art to coat at least one face of an ophthalmic lens with several coatings for imparting to the finished lens additional or improved optical or mechanical properties.
Thus, it is usual practice to coat at least one face of an ophthalmic lens, typically made of an organic glass material, with successively, starting from the face of the lens, an impact resistant coating (impact resistant primer), a scratch resistant coating (hard coat), an anti-reflecting coating and, optionally, a hydrophobic top coat. Other coatings such as polarized coating, photochromic or dying coating may also be applied onto one or both faces of the ophthalmic lens.
Numerous processes and methods have been proposed for coating a face of an ophthalmic lens and are disclosed.
US 2003/0017340 describes one process or method for transferring a coating from at least one mold part onto at least a geometrically defined surface of a lens blank comprising                providing a lens blank having at least one geometrically defined surface;        providing a support or mold part having an internal surface bearing a coating and an external surface;        depositing on said geometrically defined surface of said lens blank or on said coating a pre-measured amount of a curable adhesive composition;        moving relatively to each other the lens blank and the support to either bring the coating into contact with curable adhesive composition or bring the curable adhesive composition into contact with the geometrically defined surface of the lens blank;        applying a sufficient pressure onto the external surface of the support so that the thickness of a final adhesive layer once the curable composition cured is less than 100 micrometers;        curing the layer of adhesive composition; and        withdrawing the support or mold part to recover the lens blank with the coating adhered onto the geometrically defined surface of said lens blank.        
By pre-measured amount, one means a sufficient amount of curable adhesive composition to obtain transfer and adhesion of the coating to the lens blank.
Preferably the coating is transferred to the back surface of the lens blank (back side treatment or BST).
The curable adhesive composition not only must allow a quick and safe transfer of the coating from the support onto the lens blank surface and a good adhesion of the coating onto the lens blank surface but also must not impair the optical and mechanical properties of the resulting ophthalmic lens, such as good adhesion, haze, no application stain and resistance to thermal cracking (critical temperature).
Application stain refers to the existence of an area of increased haze level, on the lens blank, coinciding with the initial adhesive composition application area. The Haze may be observed if the adhesive composition was allowed to sit on the lens blank surface for longer than 20 seconds before undergoing the surface transfer process.
Additionally, the cured adhesive composition layer must have a refractive index close to that of the substrate.